Broadcast communications systems utilize electromagnetic signals of a frequency or range of frequencies specific to that system in order to transmit information between remote locations. The frequencies must be known to both the transmitter and receiver in order for the communication to be successful. The receiver will only recognize signals of the selected frequencies, and will treat all signals of those frequencies as the received communication. As a result, any phenomenon which modifies the transmitted signal prior to its reception will distort the information received. This is called "interference".
Electromagnetic signals of the same or similar frequencies will combine to form a new signal. As a result, multiple communications signals cannot easily utilize the allocation of frequency spectra in the same geographic region. This has led to governmental regulation to assign specific frequency spectra or "bands" to specific uses. Unfortunately, there are only a finite number of such bands available in any given geographic region, and there is an ever-increasing number of users desiring to utilize those bands.
Two such forms of communication are microwave transmission and Personal Communications Service ("PCS"). Microwave transmission are point-to-point transmission systems utilizing signals in the 1.5 GHz to 40 GHz frequency bands. The transmitted signal is aimed along a direct line and most of the energy in the signal is directed along a line-of-sight at a frequency in that band. This is made possible by the fact that both the transmitter and receiver are located at fixed locations, and hence it is simple to aim the signal.
PCS has been defined by the Federal Communications Commission in Docket GEN 90-314, June 1990. PCS may utilize signals in the 1.5 GHz to 2.5 GHz frequency bands, which overlap with the microwave transmission frequency bands discussed above. Also, PCS units are not stationary, as will be discussed below. One example of a service offered within a PCS system is a portable cellular telephone-like service. Because one station in such service is portable, it is difficult to broadcast in a narrow path due to tracking difficulties. Hence a portable PCS transmitter system such as a cellular telephone device requires a broader geographic range of transmission.
There is a need in microwave transmission systems to maintain a low error rate. The bit error rate ("BER") of such systems can be as low as 10.sup.-7 or 10.sup.-8. Presently, the microwave system uses two linear polarized signals. One of these signals is "vertically polarized", meaning that the electric field lies entirely within a vertical plane. The second signal is "horizontally polarized", meaning that the electric field lies entirely in a horizontal plane. When the two signals are perpendicular, as in this case, they do not interfere with one another. The signals are said to be "orthogonal". The two signals in the microwave system are used to improve adjacent channel discrimination. It is commonly practiced in microwave relay systems to interleave alternate radio channel frequencies on the two linearly polarized signals. It is also common to transmit two different information signals on the two linearly polarized signals. Together with the highly directional nature of the signal, this allows for an efficient and reliable transmission of data.
As discussed above, PCS systems including cellular telephone service require a far greater broadcast area than microwave systems, as the mobile unit is often located in a moving vehicle and it is difficult to precisely align a signal with a moving receiver. Although the need for a low error rate is not as critical for PCS systems as that for a microwave system, voice quality and data transmission are improved by minimizing interference to the transmitted signal.
PCS systems such as cellular telephone systems typically utilize a vertically aligned signal. As is clear from the above discussion, such a signal would interfere with any microwave signal broadcast in the same frequency band in the same geographical region. Even minor interference would raise the BER for the microwave system to an unacceptable level. Hence prior art systems do not permit PCS and microwave systems to share the same frequency band.
It would therefore be desirable to provide a communications system which allow both microwave and PCS systems to share a common frequency band with minimal interference between the signals.
Broadly, it is an object of the present invention to provide an improved broadcast communications system.
It is a further object of the present invention to provide a communications system which allows a microwave transmission system and a PCS system utilizing cellular telephone technology to share a common frequency band with minimal interference between the signals.
These and other objects of the present invention will be apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.